Test EntityManager using JUnit Mockito

I am using Junit with Mockito. I want to test EntityManager, i am getting java.lang.NullPointerException

The below is what i have tried, main class method is,

@Override
    public ReplicationPerspective buildReplicationPerspective(final String replicationDomain)
        throws ReplicationStateException {
        try {
            System.out.println("Test");
            final ReplicationPerspective localPerspective =
                this.replicationPerspectiveQuery.findReplicationPerspective(replicationDomain);

            List<String> ncdKeys = new ArrayList<String>();
            for (NodeChangeDelta ncd : this.nodeChangeDeltaQuery.findByChangeStatus(
                replicationDomain, ChangeStatus.PENDING)) {
                ncdKeys.add(ncd.getKey());
            }
            localPerspective.setPendingNodeChangeDeltaKeys(ncdKeys);

            LOGGER.debug("Local perspective is {} ", localPerspective);
            return localPerspective;
        }
        catch (Throwable t) {
            LOGGER.error("Failed to build replication perspective", t);
            throw new ReplicationStateException(t);
        }
    }

replicationPerspectiveQuery Bean file method is,

@PersistenceContext
    private EntityManager em;

@Override
    public ReplicationPerspective findReplicationPerspective(final String replicationDomain) {
        Validate.notBlank(replicationDomain);

        ReplicationPerspective perspective =
            this.em.find(ReplicationPerspective.class, replicationDomain);
        if (perspective == null) {
            this.replicationPerspectiveInitializer
                .initializeReplicationPerspective(replicationDomain);
            perspective = this.em.find(ReplicationPerspective.class, replicationDomain);
        }

        return perspective;
    }

And my test case method is,

@Test
    public void testBuildReplicationPerspective() throws ReplicationStateException {
            this.replicationStateServiceBean =
                new ReplicationStateServiceBean(null, null, null, null,
                    new ReplicationPerspectiveQueryBean(), null, null);

            this.em = Mockito.mock(EntityManager.class);
            Mockito.when(this.em.find(ReplicationPerspective.class, REPLICATION_DOMAIN))
                .thenReturn(null);

            this.replicationStateServiceBean.buildReplicationPerspective(REPLICATION_DOMAIN);
    }

I am getting NPE error in replicationPerspectiveQuery Bean file at the below line

ReplicationPerspective perspective =
            this.em.find(ReplicationPerspective.class, replicationDomain);

How to test entity manager, help me to solve.

I have also tried to mock like below but didn't work,

Mockito.when(this.replicationPerspectiveQuery.findReplicationPerspective(REPLICATION_DOMAIN)).thenReturn(null);